Water purification apparatus and units for use in laboratories and healthcare facilities are well known. Generally, they involve the reduction and/or removal of contaminants and impurities to very low levels. They typically contain a variety of technologies that remove particles, colloids, bacteria, ionic species and organic substances and/or molecules.
The levels of such contaminants and impurities can be monitored in various ways, one being to measure the total organic carbon content (“TOC”) as a measure of any remaining organic substances in the water. TOC limits for various purities or grades of water are prescribed by various national and international bodies, for example the US and European pharmacopoeias (USP/EP), the American Society for Testing and Materials (ASTM) and the Clinical Laboratory Standards Institute (CLSI).
Dedicated TOC monitors are well known in the art; see for example the monitor shown in WO99/42824A1. A TOC monitor can be located at or near the point of dispense of purified water from a water purification unit to directly confirm the TOC of the dispensed water to a user and/or service engineer. However, dedicated TOC monitors are expensive, and also require separate maintenance, adding to the CAPEX and OPEX of such water purification units. They also use some of the purified water.
EP 0 498 888 A1 describes a method of measuring the total amount of organic substances in ultra-pure water by applying ultraviolet rays to ultra-pure water whose specific resistance value has been set to a known constant. However, this arrangement requires treatment of the feed water to ensure that this value is achieved and does not attempt to measure TOC of the water being dispensed.
Alternatively, the difference in specific resistance values before and after the application of ultraviolet rays to a water stream is used to estimate the TOC of the water being dispensed.